Extract with full boil

In my experience the boilover issue is going to be the trickiest to calculate. Depending upon the amount and types of grain used I get different amounts of foaming up which leads to boilovers. Also pre-boil volume I find also varies from place to place. In a place with lower humidity you get much more loss than a place with higher humidity which affects your final boiled volume. But again the shape of your pot will affect this as the surface to air ratio will be different with different sized pots. In order to make these calculations those variables would need to be taken into account.
–
Chris PlaisierFeb 25 '13 at 21:06

Awarding the bounty to MalFet as his answer was the closest to what I asked for. Mdma's answer was nevertheless very insightful on clarifying many issues on calculating these volumes.
–
Cleber GoncalvesMar 1 '13 at 12:20

Grain absorption is easy to estimate. I have always used 0.1 gallons of absorption per 1 pound of grain, or roughly .8 liters per kilo. This is the figure used by most of the software, and it has always been very accurate for my purposes.

Boilover varies significantly by the shape of the vessel and the nature of the heat source. I like to have at least 2-3 gallons (~8-12 liters) of spare space. You can get by with less (1-2 gallons; ~4-8 liters), especially if you are willing to use something like Fermcap to limit foam production that would lead to a boilover. There is no absolute answer here, but a common rule of thumb is to buy a boil kettle that is double the volume of your intended batch size

Your pre-boil volume is simply your batch size plus the amount of water you expect to lose due to boil off. The exact amount that you will lose to boil off is extremely complex, and it will depend by a number of different factors that cannot be sufficiently measured. Your best bet is to run a couple of tests to see what your average boil off will be. Until you figure out the values for your system, it is usually best to assume a low boil off, because it is always much easier to add water than to take it away.

I got the 1.65l/kg from brew365, supposedly it comes from Ray Daniel's 'Designing great beers' book. As noted on the article some other references point to the same number you use, I picked the bigger number to stay on the safe side. What is the size of your boil kettle and the batches you do? 8-12 liters sound quite a lot for a 29.5 liter vessel (1/3 of the size?)
–
Cleber GoncalvesFeb 20 '13 at 9:31

Right, I'm not sure where Ray Daniels got his figures, but they're exactly double what everyone else uses. Certainly no harm will come from assuming you need more space. A general rule of thumb is that you want a boil kettle twice your batch size. In the US, most people use 10 gallon kettles for 5-6 gallon batches, though some get away with 8 if they watch closely for boilovers. Alternately, you can use a foam reducer like fermcap.
–
MalFetFeb 20 '13 at 12:23

Do you mean 5-6 gallons of final volume on the fermentor? Boil volume will be more then? 10 gallons will be about 38 liters. That is 8.5 more than the minimum volume of 29.5 I had calculated to boil kettle. I wonder if that is really a must, I have never had a boil-over, but the one time I saw the boil actually grown a bit with foam, it didn't go more than some 5 centimeters (about 5 liters on the kettle). Do your have frequent boilovers? How much your wort 'grow' during that? Is there an style that is more prone to it?
–
Cleber GoncalvesFeb 20 '13 at 13:19

Yes, that's 5-6 (19-23L) gallons of final volume, which would be 6-7 gallons (23L-26.5L) of preboil volume. I don't have boilovers because I use a moderately powered burner and a big pot, but in certain circumstances you can get a very large boilover (especially during hot break). It's just a trade off; a smaller pot will work but might require more vigilance on your part.
–
MalFetFeb 20 '13 at 13:30

I have added a few more details to my answer. If this doesn't answer your question, perhaps you could be more specific about what is unclear? There will be no precise formula to determine boiloff rate or necessary boilover slack, simply because these things rely of a variety of factors that cannot all be predicted from the outside.
–
MalFetFeb 22 '13 at 12:19

While the formulae can give you some insight into the appropriate sizes, there are quite a few unknowns. I would instead take into account the common wisdom for kettle sizes.

The general rule of thumb is to have the boil kettle around twice the volume of your target batch. I brew 38 liter/10 gallon batches and use a 75 liter/20 gallon blichmann kettle. Pre-boil volume is typically just shy of 3/4 of the kettle capacity so boilovers are seldom, but it has happened once. The absolute minimum would be 1.5x your batch size, but the closer you get to 2x batch size the more convenient and pleasurable it will be to brew. Even with 2x batch size, you cannot totally eliminate boilovers, but they are seldom. As you reduce the kettle size, you have to be more attentive to boilovers, so bigger is definitely better here if you can accommodate it.

For the MLT, you may go smaller, down to 1.2x the batch size if you are sure you won't be making anything above 1.060. In my experience, the volume in my mash is never more the final batch size for regular strength beers.

For the HLT, you generally need about twice as much water as the final batch volume. My HLT is 2x my batch bolume - fill the kettle up at the start of brewday and use almost all of it - maybe a gallon left at most. Having more water also helps with the HERMS heat exchange. You could in principle go smaller if you're not using HERMS or if you're prepared to heat the water multiple times, e.g. heat the strike water, then any additional infusions, and then the water for the sparge. The HLT gives you the most flexibility and could be as small as 0.75x your batch size, but the smaller size will make the system less convenient.

Summarizing these figures for minimum sizes and useful range:

BK: 1.5-2 x batch size = 30-40l. (Go for the largest you can accommodate.)

MLT: 1.2+ x batch size = 24-40l.

HLT: 0.75-2 x batch size = 15-40l.

These figures tie in well with your original estimates.

EDIT: I'm assuming you'll be mounting an outlet valve on the kettles. If so, I doubt you'll be able to store the kettles inside each other. It may be better instead to just get 3 kettles the same size 35-40l and stack them vertically when not in use.

EDIT 2: If you don't have the space for 3 kettles now, when why not just go with a single 40l kettle and do BIAB until you have space. I have a 3 kettle setup, but sometimes wish I could do the same with 1 - they take up a lot of room (see my profile pic.) The danger of reducing the size down to the absolute smallest so that the kettles can be stacked is that you spend quite a bit on the kettles, yet you may still end up with a hampered brewday due to inadequate kettle sizes. Also you'll need a pretty big stove top/hob to have room for all three kettles, if you're brewing in the kitchen. If you go with a single kettle solution now, you'll be able to brew all grain without needing space for 3 kettles, and also be sure you have stovetop space and power to be able to brew effectively. You can then always add 2 additional kettles later when your circumstances change.

Good point on the valves, had overlooked that. Unfortunately I just don't have the space for storing 3 40L kettles, even vertically, so will have to get creative there... Perhaps I can find a small valve.
–
Cleber GoncalvesFeb 24 '13 at 16:18

Good advice, I'm also looking at that option (actually stated that on my question) but I wanted to explore the possibility of having a full 3 kettle setup. I also feel that question can be useful for others trying to figure out which kind of equipment they need (hence I included the figures for BIAB, Extract and no sparge).
–
Cleber GoncalvesFeb 24 '13 at 19:58

I don't think trying to formalize it is a good idea - there are simply too many variables to make this generally applicable: grains have different absorption rates (flaked vs malt), evaporation rates on the shape of the vessel and power to the boil can vary from 5%-20%, and losses to trub and hop absorption varying by recipe, by pH of the boil, on lauter filtering efficiency, and I'm pretty sure there are more variables that I've not thought of! Also not all brewdays go smoothly, sometimes you need extra capacity, such as when using more sparge water to make up for a low mash yield.
–
mdma♦Feb 25 '13 at 1:51

Do you really have 5l deadspace in your boil kettle? - that's 20% wastage on a 20l batch just there, which is why the 1.5x batch size seems so close to the liquid level. I brewed 20l batches for a year in a 25l kettle.
–
mdma♦Mar 1 '13 at 18:13

My friend have a 33l kettle, the valve is installed 5cm from the bottom, which gives about 5l deadspace, looking back it could probably be installed lower since most of the time there is a good amount of beer left on top of the trub, but some brews produce a lot of trub and on these cases the extra centimeters come handy.
–
Cleber GoncalvesMar 2 '13 at 22:48